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9262 



BUREAU OF MINES H\3 

INFORMATION CIRCULAR/1990 




Computer-Automated Measurement- 
and Control-Based Workstation 
for Microseismic and Acoustic 
Emission Research 



By F. M. Boler and P. L. Swanson 




«?* w % 



80 

\ YEARS g 

**AU OF ^ 



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¥ 



U.S. BUREAU OF MINES 
1910-1990 



THE MINERALS SOURCE 



Mission: As the Nation's principal conservation 
agency, the Department of the Interior has respon- 
sibility for most of our nationally-owned public 
lands and natural and cultural resources. This 
includes fostering wise use of our land and water 
resources, protecting our fish and wildlife, pre- 
serving the environmental and cultural values of 
our national parks and historical places, and pro- 
viding for the enjoyment of life through outdoor 
recreation. The Department assesses our energy 
and mineral resources and works to assure that 
their development is in the best interests of all 
our people. The Department also promotes the 
goals of the Take Pride in America campaign by 
encouraging stewardship and citizen responsibil- 
ity for the public lands and promoting citizen par- 
ticipation in their care. The Department also has 
a major responsibility for American Indian reser- 
vation communities and for people who live in 
Island Territories under U.S. Administration. 



Information Circular 9262 

! 



Computer-Automated Measurement 
and Control-Based Workstation 
for Microseismic and Acoustic 
Emission Research 



By F. M. Boler and P. L. Swanson 



UNITED STATES DEPARTMENT OF THE INTERIOR 
Manuel Lujan, Jr., Secretary 

BUREAU OF MINES 
T S Ary, Director 






Library of Congress Cataloging in Publication Data: 



Boler, Frances M. 

Computer-automated measurement- and control-based workstation for micro- 
seismic and acoustic emission research / by F. M. Boler and P. L. Swanson. 

p. cm. - (Information circular / Bureau of Mines; 9262) 

Includes bibliographical references. 

1. Rock bursts-Forecasting-Data processing. 2. Microseisms-Measurement- 
Data processing. 3. Microcomputer workstations. I. Swanson, P. L. (Peter L.) 
II. Title. III. Series: Information circular (United States. Bureau of Mines); 9262. 

TN295.U4 1990 [TN317] 622 s-dc20 [622'.28] 90-1943 CIP 



CONTENTS 

Page 

Abstract 1 

Introduction 2 

Acoustic emission-microseismic workstation 2 

Data acquisition hardware 3 

Trigger circuitry 4 

Computer hardware 4 

Data acquisition software 5 

Data analysis software 5 

Summary 9 

References 9 

ILLUSTRATIONS 

1. Microseismic activity at hardrock mine 2 

2. CAMAC data acquisition workstation 3 

3. CAMAC crate 4 

4. Process flow diagram 6 

5. Example of monitor display of microseismic event 7 

6. Three-dimensional graphics display 8 





UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT 


Hz 


hertz Mb 


megabyte 


Kb 


kilobyte MHz 


megahertz 


kHz 


kilohertz s 


second 


m 


meter 





COMPUTER-AUTOMATED MEASUREMENT- AND CONTROL-BASED 

WORKSTATION FOR MICROSEISMIC AND 

ACOUSTIC EMISSION RESEARCH 



By F. M. Boler 1 and P. L. Swanson 1 



ABSTRACT 

The U.S. Bureau of Mines has configured a flexible data acquisition and analysis workstation, which 
incorporates control and analysis software written in-house for use in mining research experiments 
conducted in the laboratory and in operating underground mines. This data acquisition system combines 
computer-automated-measurement-and-control (CAMAC) modular instrumentation with a Unix-based 
graphics workstation. The data acquisition front-end hardware can be interfaced to almost any 
computer. CAMAC instrumentation modules adhere to mechanical, electrical, and digital interface 
standards and can be custom designed or purchased off the shelf. Communications along the CAMAC 
bus are regulated by the crate-controller module, which contains the only hardware link to the computer 
interface. Software support for the data acquisition and analysis workstation consists of a series of 
functions and programs written in the "C" programming language designed to provide flexible and fast 
data acquisition, processing, and analysis. The user can interactively control the digitizers and other 
CAMAC modules by sending standard CAMAC read, write, and command function codes from a menu- 
driven program. Automated data collection is achieved through software that responds to hardware 
interrupts triggered by acoustic emission or microseismic events. Graphics display routines are used to 
plot part or all of the data and provide the ability to process and store signal information selectively. 



^eophysicists, Denver Research Center, U.S. Bureau of Mines, Denver, CO. 



INTRODUCTION 



To mitigate hazards associated with rock failure in 
underground mines, the U.S. Bureau of Mines conducts 
investigations of rock failure behavior both in laboratory- 
scale tests and in direct monitoring at underground mines. 
In this Bureau report, a CAMAC-based data acquisition 
system is described, which is used to study mine and 
laboratory rock failure problems. 

Rock bursts are abrupt rock mass failures within mines 
that cause damage to the mine workings. Rock bursts 
have been observed to be related to the patterns of micro- 
seismic events in time and space (I). 2 Rock bursts and 
other microseismic activity are a consequence of the inter- 
action of the mining extraction process with the existing 
geologic structure and the ambient stress field. Figure 1 
shows microseismic events generated in response to 
routine mining activity near a metal ore vein, which is 
offset several meters by a steeply inclined fault (Coeur 
d'Alene district, northern Idaho). The microseismicity 
patterns illustrate the influence that certain fault structures 
can have on the mechanics of deformation in the mining 
environment. 

Individual microseismic events are a manifestation of 
the rock structure-mine geometry-stress interaction leading 
to a variety of modes of rock failure. Both the time-space 
pattern of microseismic events and the seismic waveforms 
contain information about the processes involved in failure. 
To retrieve the information from waveforms at individual 
accelerometers, the entire waveform of the seismic signals 
must be recorded. Then, in a manner equivalent to 
seismogram analysis for earthquakes, mine-related 
microseismic activity can be analyzed to obtain the event 
location and focal mechanism (orientation of the fault slip 
plane and slip direction). The spectral content of the 
waveforms can be analyzed to obtain stress drop, rupture 
strength, and source dimensions. 




LEGEND 

Microseismic events 
— 4300 level 
--- 4600 level 
^b Ore vein 
\ Faul t strike 
**** Instrumentation 

borehole (4300) 



o 



10 



Scale, m 



Figure 1 .-Microseismic activity at hardrock mine. A, Plan view 
of 4300 and 4600 levels (1 ,300 and 1 ,400 m) below surface at the 
study site; B, microseismic activity near 4300 level, which 
indicates activation of preexisting fault 



The same methods of analysis can be applied to 
microcrack-scale failure events (acoustic emissions), which 
occur in laboratory rock specimens under stress. To cap- 
ture and analyze the typically large numbers of events 
(perhaps hundreds per day in a given mine working area 
or hundreds per hour in laboratory acoustic emission 
work), computer control of digital data acquisition and 
semiautomated computer processing and analysis at the 
underground fieldsite or in the laboratory is necessary. A 
workstation with these features has been configured, con- 
trol and analysis software has been written, and both have 
been tested in a field situation. 



ACOUSTIC EMISSION-MICROSEISMIC WORKSTATION 



Five fundamental requirements were involved in the 
selection of hardware for the acoustic emission 
(AE)-microseismic (MS) workstation. These were the 
ability to (1) acquire multichannel (e.g., 32 channels) 
microseismic data at sampling rates of 100 kHz per chan- 
nel and event rates of several tens of events per minute; 
(2) manage multichannel (e.g., 8 channels) AE-data acqui- 
sition at sampling rates of 10 MHz or more per channel 
and event rates of several events per second; (3) acquire 
quasi-static measurements of stress, displacement, and 
other physical parameters on an arbitrarily large number 
of additional channels; (4) acquire, display, and analyze 
data in a multiuser, multitasking environment. The fifth 

Italic numbers in parentheses refer to items in the list of references 
at the end of this report. 



requirement was that the hardware had to be rugged, reli- 
able, compact (able to fit into a single standard equipment 
rack), and transportable. Although there are computer 
systems with data acquisition boards that incorporate char- 
acteristics capable of accomplishing requirements 1, 3, 4, 
and 5, no affordable, easily expandable system was found 
to be capable of handling the high analog-to-digital (A-D) 
conversion and data transfer rates of requirements 1 and 
2 simultaneously, and meet requirement 5 as well. A sys- 
tem capable of performing as required, comprised of three 
essential components, was configured. These components 
are (1) a CAMAC system for signal conditioning, A-D 
conversion, and short-term data storage; (2) a trigger 
circuit; (3) a computer graphics workstation for communi- 
cation with the CAMAC and data analysis. Each of these 
components is discussed in greater detail. 



DATA ACQUISITION HARDWARE 

CAMAC instrumentation hardware was selected for a 
variety of reasons, a major one being its modularity. 
CAMAC instrumentation was originally developed for 
experiments performed in the nuclear physics field (2); it 
is a well-established technology. CAMAC modules, manu- 
factured by over 50 companies, adhere to international 
digital interface and modular instrumentation standards 
(3). A CAMAC module is any circuit board of nearly any 
function (A-D conversion, memory, signal conditioning, 
stepper motor controller are examples) that is built to slide 
into a CAMAC crate. The CAMAC crate supplies power 
and a backplane (CAMAC bus) for intermodule and 
CAMAC-to-computer communication. Modules from 
various manufacturers easily function together in a 
CAMAC crate under computer control. The specific 
computer model and the interface used are not dictated by 
the CAMAC system, although the crate controller 



generally must be selected with the computer interface 
capabilities in mind. 

The requisite range of digitizing rates (10 Mhz for AE, 
100 kHz for MS) is readily obtained by a selection of 
appropriate A-D conversion modules. Additional channels 
and memory for A-D conversion are easily incorporated. 
As improved modules are developed, the system can be 
upgraded. Also, user-developed circuitry can be incor- 
porated into custom CAMAC modules. 

Figure 2 shows the CAMAC and computer hardware 
configured for the MS and AE experiments. The system 
is currently configured for 32 accelerometers sampled at 
up to 100 kHz per channel with 12-bit resolution, with a 
1-megasample (2-Mb) memory for MS monitoring and for 
8 AE transducers at up to 20-MHz sampling rate per 
channel with 8-bit resolution with 8-kilosample memory 
per channel. The AE and MS data acquisition transfer 
cycle is started upon receipt of a stop trigger from cir- 
cuitry that is separate from the CAMAC system. Trigger 



Tr igger 
logic 





Programmable 

gai n 
amp I i f iers 



12-bit 

ADC 

100 kHz/ 

channel 



Microsei smic 

transient 

recorder 

control ler 

9 



Local 
memory 
1 mega- 
sample 



Multi- 
channel 
DVM 



CAMAC BUS. 



IEEE-488 

CAMAC bus 
control ler 



Hard disk 



3-D color 
graphics 
moni tor 






CPU 


16 


Mb RAM 


6803C 


processor 




UNIX 



IEEE-488 



Other graphics 
terminals 



Tape storage 



Figure 2.-CAMAC data acquisition workstation for AE-MS full-waveform capture, stress-displacement measurement, and data 
analysis and display. DVM-dlgltal voft meter, LAN-local area network, ADC-analog-to-digital conversion, CPU-computer processing 
unit, RAM-random access memory, UNIX-operating system. (IEEE-488 is also known as GPIB.) 



circuitry inputs are analog accelerometer signals (MS) or 
ultrasonic transducer signals (AE). In general, part of the 
signal before and after the first arrival is captured on all 
channels by controlling the relative amounts of pre- and 
post-trigger recording. The system is configured with a a 
general purpose interface bus (GPIB) interface for 
computer-CAMAC communication (4). Additional mod- 
ules for signal conditioning and for quasi-static rock 
mechanics measurements of loads and displacements are 
configured with this system. The present configuration of 
the instrumentation modules and CAMAC crate is shown 
in figure 3. 

Multiuser access to the crate is also possible. For 
example, one user can monitor rock mechanics instru- 
mentation while another monitors the MS acquisition. 
Also, processor priority and interface locking are features 
of the operating system (Unix) and the interface library 
software, so that multiple processes accessing the crate can 
be in action concurrently. 

TRIGGER CIRCUITRY 

The external trigger circuitry used for the MS data 
acquisition is a modification of the design by Sonder- 
geld (5). This circuitry provides a transistor-transistor 
logic (TTL) pulse to the microseismic transient recorder 



controller (fig. 2). Noise discrimination is accomplished in 
three ways. First, the signal frequency must exceed a 
certain set frequency. This prevents triggering by distant 
events that are unrelated to the study area and by low 
frequency mining production noise. Second, the signal 
must contain a minimum number of cycles. This allows 
elimination of certain electromagnetic noise signals, which 
are typically of short duration and few cycles relative to 
MS events. Third, as currently configured, up to four 
channels can be required to meet the discrimination 
criteria with a minimum delay time between arrivals at 
each channel. This guarantees that non-EM signals be 
large enough to trigger up to four channels. The dead 
time between triggers is also adjustable. 

A similar configuration of AE triggering is possible. A 
means of insuring that all AE channels receive the stop 
trigger at the same time (within some tolerance) would 
need to be added to the circuitry described previously. 

COMPUTER HARDWARE 

The multitasking acquisition and analysis environment 
is provided by a Motorola Corp.-68030 based Unix work- 
station (fig. 2). The system is configured with 16 Mb of 
random access memory (RAM), and a 130-Mb hard disk 
drive. For display of microseismic event locations and 



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Figure 3.-CAMAC crate configured with acoustic emission, microseismic, and signal conditioning modules. 



focal mechanisms overlaid on mine structure and geology, 
a hardware graphics accelerator is utilized. This allows 
examination of data in three dimensions with a continuous- 
ly variable point of view, greatly enhancing the ability to 
make correlations of microseismic activity with mine 
structure and geology. 



An important aspect of a research-grade multichannel 
full-waveform microseismic system is the ability to archive 
large quantities of data for later analysis. During field 
acquisition, the unattended tape storage capacity of 
the system is 536 Mb or about 10 typical days' worth of 
data. 



DATA ACQUISITION SOFTWARE 



The software developed in-house 3 for data acquisition 
is menu-driven. The program accomplishes its tasks via 
function calls that send instructions to and receive data 
from the CAMAC. Some of these function calls are also 
configured as system level commands. Figure 4 shows a 
general process flow diagram for data acquisition. One 
of the main tasks of the program is configuring the 
CAMAC modules. For the A-D converters, configuration 
parameters include number of channels, number of 8-bit 
(AE) or 12-bit (MS) samples per channel, sampling rate, 
number of pretrigger samples to retain, and individual 
channel gains for the signal conditioners. CAMAC con- 
troller parameters include data transfer mode (block mode 
transfer versus word-by-word transfer) and individual 
module lockout (from system interrupt capability). 

When an event occurs, the trigger circuitry issues the 
stop trigger to the continuously sampling CAMAC trans- 
ient recorders. (A stop trigger for the converters can also 
be sent using software control.) Once triggered, the 
CAMAC controller module, which oversees transmissions 



on the CAMAC bus, notifies the computer that data are 
ready for transfer. Typically, the program is setup to 
monitor the CAMAC in the background. This allows 
other tasks, such as plotting or event location to take place 
in the foreground. The background program, which has 
the highest processor priority, handles transfer of data 
from CAMAC to memory and then to disk files (demulti- 
plexing channels if necessary in the process), followed by 
re-arming of the A-D converters for the next microseismic 
event. It takes 2 to 3 s to transfer a 200 Kb event from 
the microseismic transient recorders and re-arm the 
system. A tape-archiving option allows tape storage to 
take place once the disk has been filled. 

Display of acquired data is required during setup of 
data acquisition parameters such as digitizing rate or 
conditioning gains. The program allows immediate 
plotting of acquired events, with options for plotting a 
subset of channels or modifying the vertical and horizontal 
scaling. Figure 5 shows an example plot of a microseismic 
event as displayed during data acquisition. 



DATA ANALYSIS SOFTWARE 



The data analysis software has been designed to take 
advantage of the interactive graphics and graphics 
acceleration capabilities of the workstation. A large part 
of analysis time involves interactive picks of arrival time 
and first-motion polarities from displayed waveforms. 
Ease of use of plotting programs and picking software is 
one of the features of the analysis programs developed 
in-house. 

An extensive description of the acquisition character : 
istics for each waveform is stored as a header record com- 
posed of C-language structures. Various pieces of in- 
formation stored in the header are required for different 



aspects of the analysis program, such as event plotting, 
event location, and signal processing. The header record 
serves as both a convenient stand-alone record of informa- 
tion about the station and/or event and as an organized 
means of storing plotting parameters or processing in- 
formation about each waveform. 

The hardware graphics engine (boards separate from 
the computer processor unit (CPU) that accelerate graph- 
ics instruction to the monitor) is used for display of micro- 
seismic event locations and mechanisms in time and space. 
These displays also incorporate geology and/or mine struc- 
ture for cross-reference (fig. 6) 4 . 



3 The authors acknowledge the contribution of Robert Rozen, 
software engineer, of Vanguard Technologies, Denver, CO, who wrote 
the original version of the data acquisition and display code, under 
contract to the Bureau. 



4 The authors acknowledge the contribution of Louis Estey, 
Geophysicist, Ground Control Division, U.S. Bureau of Mines, who 
wrote the three-dimensional mine structure display software. 



DATA ACQUISITION PROGRAM FLOW 



SETUP 



CAMAC SETUP 

(DIGITIZING RATE, 
NUMBER OF CHANNELS,...) 




PROGRAM PARAMETER SETUP 

1.) HARD DISK SPACE, TAPE SPACE 

2.) HEADER PARAMETERS 

3.) ACQUISITION AND STORAGE MODE 





BACKGROUND ACQUISITION 



EVENTS 




BACKGROUND 



ON INTERRUPT FROM 
CAMAC: 



1.) TRANSFER DATA 
2.) WRITE DISK FILE 
3) CHECK DISK SPACE 



I 



FOREGROUND 



AVAILABLE FOR 

PLOTTING OR 

ANALYSIS... 



FOREGROUND ACQUISITION 




EVENTS 



FOREGROUND 

ON INTERRUPT FROM 
CAMAC: 

1.) TRANSFER DATA, 
DEMULTIPLEX 

2.) PLOT 



ON DISK FULL: 

1 .) SUSPEND INTERRUPT SERVICING 

2.) WRITE TAPE 

3.) RETURN TO PREVIOUS ACQUISITION 



ON TAPES FULL 

1.) FILL DISK 

2.) SUSPEND INTERRUPT SERVICING 



— ^0*0 



3.) OPTION TO SAVE DATA 
TO DISK FILE 

4.) CHECK DISK SPACE 



Figure 4.-Process flow diagram for data acquisition software. 




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SUMMARY 

A computer workstation has been described with system is flexible by virtue of the CAMAC modularity and 
associated data acquisition and analysis hardware, which expendability. The multitasking, multiuser computer work- 
has been configured and used for acoustic emission and station serves the needs for simultaneous data acquisition 
microseismic waveform recording. The data acquisition and analysis, or for multiple analysis tasks. 



REFERENCES 

1. Brady, B. T. Prediction of Failures in Mines-An Overview. 4. IEEE-488. Digital Interface for Programmable Instrumentation. 
BuMines RI 8285, 1978, 16 pp. ANSI/IEEE Standard 488, 1978, 83 pp. 

2. Cleary, Robert T. The IEEE-583 Bus CAMAC, A Versatile 5. Sondergeld, C. H. Effective Noise Discriminator for Use in 
Interface Standard, TN-103. Kinetic Systems Corp., Lockport, IL, 1986, Acoustic Emission Studies. Rev. Sci. Instrum., v. 51, 1980, pp. 1342- 
12 pp. 1344. 

3. IEEE-583. Modular Instrumentation and Digital Interface System 
(CAMAC). ANSI/IEEE Standard 583, 1982, 81 pp. 



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